Site stormwater: where your roof water goes, and why it has to go there

Gutters and downpipes get water off the roof. What happens after that is just as important, and almost entirely hidden. The pipes run underground, the pits get covered by garden beds, and nobody thinks about them again until water comes through a wall, a slab starts to move, or a neighbour knocks on the door about the water flowing onto their property.

This article covers where a property's stormwater is allowed to go, what the code and standards require of the drainage that gets it there, why the rules are set the way they are, and how we build and inspect it.

What the code requires

For houses, the National Construction Code (NCC) 2022 covers site drainage in the Housing Provisions, Part 3.3 Drainage, which was new in the 2022 edition. It requires three things around a Class 1 building:

• Surface water drainage. The ground around a slab-on-ground house must fall away from it, in most cases by at least 50 mm over the first metre, and the slab must sit high enough above the finished ground, generally 150 mm, with lower figures allowed next to properly sloped paving or in well-drained areas. Under a suspended floor, the ground must be shaped so water does not pond beneath the building.
• Subsoil drainage, where site conditions need it. Where groundwater or seepage is expected, subsoil drains must be laid to a uniform fall of at least 1:300 and discharge to a pit that can be cleaned.
• Stormwater drainage. Where a system is required, the position and manner of discharge must be to the satisfaction of the appropriate authority, and the system must be designed so that overflow in heavy rain cannot flow back into the building.

The installation itself is covered by AS/NZS 3500.3, the plumbing and drainage standard for stormwater, whose 2021 edition is referenced in NCC 2022. In NSW, stormwater drainage is licensed plumbing and drainage work, and the drainer should issue a certificate of compliance for it.

Where the water is allowed to go

"To the satisfaction of the appropriate authority" is doing a lot of work in that clause. In practice it means your council, and sometimes Sydney Water, decides the legal point of discharge for your site. It is usually one of:

• the street kerb and gutter, through a kerb outlet, for sites that fall towards the street;
• a council pit or pipe in the street or a drainage reserve;
• an inter-allotment drainage easement, a pipe running through neighbouring lots that your site has a registered right to drain into, common for lots that fall away from the street.

What it is never, without a legal right to do so, is your neighbour's yard. A downpipe that just spills onto the ground at the low side of the block, or a pipe that daylights at the back fence, sends your roof water onto someone else's property. That is a common source of neighbour disputes, and depending on the circumstances it can expose an owner to a claim for the damage it causes. If water from your site is reaching a neighbour's property, get it fixed properly and, if a dispute has started, get legal advice.

On-site detention: why some sites have to hold water back

Roofs, driveways and paving turn rain that used to soak into the ground into fast-moving runoff. Every new house or extension adds a little more. The public drainage system downstream was designed for a certain amount of flow, and the more hard surface there is upstream, the more often it overflows.

On-site stormwater detention (OSD) is the answer many authorities require: a tank, an underground chamber, or a shallow depression in a driveway or yard that temporarily stores stormwater during a storm and releases it slowly through a small, carefully sized outlet. Sydney Water's on-site stormwater detention guide, which applies where a property connects to a stormwater asset owned by Sydney Water or within the City of Sydney local government area, explains it plainly: OSD exists so that development does not increase flooding downstream. Its guide sets out that:

• the design must be prepared by a qualified professional and certified by a chartered civil engineer;
• the system is sized to a permissible site discharge, the most the downstream system can take from your site, and a site storage requirement, the volume needed to hold the excess back;
• a single residential dwelling, a small site, or a refurbishment keeping the existing drainage may not need one;
• once built, the owner must maintain it, and where it drains to a Sydney Water system, an agreement covering maintenance is registered on the property title so the obligation passes to future owners.

Most Sydney councils have their own OSD requirements in their development control plans, with different thresholds and design rules, so the answer for your site depends on your council and catchment. Find out early. OSD affects the site layout, the levels and the budget, and it is far easier to design in than to retrofit.

Rainwater tanks are not the same thing. A tank that is kept full for reuse may satisfy a BASIX water target, as covered in our post on what 7-star BASIX means, but a full tank cannot also hold back storm runoff. Where both are needed, they are designed as separate volumes or as a combined tank with a dedicated detention zone.

Why the rules are set this way

Water that stays near a building causes damage that is slow and expensive. The NCC's falls and slab heights exist because ponding water against a slab edge or under a floor feeds rising damp, rots timber, corrodes fixings and, on reactive clay sites, wets the soil under one part of the footing and not another. Our posts on footings and site classification and what a damp patch is telling you explain how that shows up.

Overflow has to have somewhere safe to go. A pipe sized for ordinary rain will be overwhelmed by an intense storm. That is why the code requires that overflow cannot run back into the building: when the pipes cannot cope, the water needs a planned path across the surface, away from doors and floor levels, not through them.

One site's runoff is another site's flood. Discharge points and OSD exist because water does not stop at the boundary. The authority controls where it goes because it is managing the whole catchment, not just your block.

It is buried, so it has to be checked before it is covered. In NSW, one of the critical-stage inspections a certifier must carry out on a house is before covering stormwater drainage connections. As we explain in our post on hold points and the handover record, once the trench is backfilled, nobody can see whether the pipes are connected, graded and bedded properly.

Where site drainage commonly goes wrong

• Downpipes not connected at all, or connected to a pipe that goes nowhere, such as a "rubble pit" in clay soil that fills in the first storm and then overflows next to the footing.
• Falls in the wrong direction. Paving or garden beds laid back towards the house, often after handover, so water runs to the slab instead of away from it.
• Garden beds and paths built up against the wall, burying weep holes and bridging the damp-proof course and termite barrier.
• Charged downpipe systems without maintenance. Where downpipes must run underground and rise again to a high discharge point, the pipes stay full of water between storms. Leaves and silt settle in the low point and the system slowly chokes unless there are accessible flushing points and they are used.
• Pits buried and forgotten. Inspection openings and pits covered by turf or pavers, so blockages cannot be cleared without digging.
• OSD outlets blocked or altered. A grate missing, an orifice plate removed to "improve drainage", or a detention area filled in during landscaping. All of these defeat the system and, where it is registered on title, can breach the owner's obligations.
• Unauthorised discharge. Roof water piped to the boundary, or into the sewer, which is not permitted.

Questions worth asking

1. What is the legal point of discharge for this site, and has it been confirmed with council?
2. Does the site need on-site detention, and who is designing and certifying it?
3. Is the system charged anywhere, and where are the flushing and inspection points?
4. Who is the licensed drainer, and will I receive the certificate of compliance and as-built drainage plan?
5. How do the finished ground levels and paving fall around the house?
6. If there is OSD, what maintenance does it need, and is there an obligation on the title?

How we build it

• We confirm the discharge point and any OSD requirement at design stage, before levels and layout are fixed, so drainage is designed in rather than squeezed in.
• We use licensed drainers and work to AS/NZS 3500.3, with the certificate of compliance handed over at the end.
• We hold before backfill. Pipes, grades, bedding, connections and pits are checked and photographed before trenches are covered, alongside the certifier's critical-stage inspection.
• We set out surface falls deliberately, around the slab and across paving, and we flag landscaping plans that would undo them.
• We give you an as-built drainage plan showing where pipes, pits, flushing points and any OSD components are, so the system can be maintained by whoever owns the property next.

See our Quality & Building Compliance policy for how we approach inspections generally. If you are planning a new build or extension and want to understand what your site's drainage will involve, get in touch.

Sources

• NCC 2022 Housing Provisions, Part 3.3 Drainage, Australian Building Codes Board
• On-site stormwater detention guide, Sydney Water (June 2020)
• Environmental Planning and Assessment (Development Certification and Fire Safety) Regulation 2021, section 61, NSW Legislation

This article is general information, not engineering or legal advice for your site. Discharge points, on-site detention and drainage requirements are set by your council and, in some areas, Sydney Water, so confirm them with your designer, hydraulic engineer and certifier. Full website disclaimer